To meet various requirements in different environments, various lights, such as desk lights for reading and writing, wall lights for illuminating the wall or artifacts, streetlights for night illumination, emergency lights installed at important evacuating passages or locations, or even car lights, have been developed. Due to different uses, their structures and configurations differ greatly such that they can not be substituted for each other. Each light thus has only a limited degree of usage.
More importantly, some situations in which illumination is needed to exhibit their features do not have specific illuminating lamps. As a result, other lights are borrowed informally for their illumination. For instance, a compact disk (CD) box for accommodating CDs, video CDs (VCDs), digital versatile disks (DVDs) has no specific illuminating lamps, external light is used for illumination instead. A photograph frame is another example having no specific illuminating lamp to exhibit figures in the photograph.